This invention relates to a transparent crystallized glass for use in a front window or an inspection window of a combustion apparatus for burning oil, coal, gas, wood, or the like, i.e., a space heater, a heating furnace, an annealing furnace, and so on.
The function of a front window in a space heater or the like is to enhance a space-heating effect by transmitting heat radiation emitted from a flame inside the heater to the outside as well as to increase visual warmth by allowing the flame to be visible. On the other hand, the function of an inspection window is to allow the burning condition of the flame to be observed from the outside. These windows must be resistant against a high temperature produced from the flame and heat shock at the ignition. Therefore, a window glass material used in the windows of such combustion apparatus is required to be transparent as well as to be low in thermal expansion and high in mechanical strength so that the heat resistance and the heat-shock resistance are excellent.
At present, as the material for the windows of the space heater, use is made of a borosilicate glass, a silica glass, and an Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass. However, the borosilicate glass is not sufficient in heat resistance and heat-shock resistance while the silica glass is excellent in thermal characteristics but is expensive. On the other hand, the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass has a small coefficient of thermal expansion and a high mechanical strength so that the heat resistance and the heat-shock resistance are excellent, and it can be produced at a relatively low cost. Therefore, this glass is widely used.
However, the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass has a problem that, when it is placed in a combustion atmosphere, the inner surface, that is, the burning-side surface of the window is subjected to chemical corrosion to produce microcracks which result in a considerable decrease in transparency and mechanical strength.
The reason why the above-mentioned problem occurs is as follows. In the combustion atmosphere of the combustion apparatus for burning oil, coal, gas, wood, or the like, SOx produced from sulfur contained in such fuel is present and reacts with H2O produced by combustion to produce H2SO4. Hydrogen ions produced from H2SO4 cause an ion exchange reaction with lithium ions in crystals of the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass to cause shrinkage in volume of the crystals, resulting in occurrence of cracks.
In order to prevent the above-mentioned problem, use is made of a technique of forming a coating film such as SiO2 on the surface of the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass. However, this technique is not preferable because the production cost becomes high. Alternatively, it is possible to suppress the occurrence of the ion exchange reaction with the hydrogen ions by reducing the content of the lithium ions in the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass. However, there arises a problem that the transparency is deteriorated and the coefficient of thermal expansion is increased.
It is therefore an object of the present invention to provide an Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass which will not produce microcracks even if it is exposed to an atmosphere containing H2SO4 produced by a fuel such as oil, coal, gas, wood, or the like for a long time, as well as a crystallizable glass for obtaining such crystallized glass.
As a result of various experiments, the present inventors have found out that xcex2-OH contained in an Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass plays an important role in suppressing the occurrence of microcracks due to an ion exchange reaction with hydrogen ions, and thus have achieved the present invention.
Specifically, an Li2Oxe2x80x94Al2O3xe2x80x94SiO2 transparent crystallized glass according to the present invention is characterized in that xcex2-quartz solid solution is produced as main crystals, an average coefficient of linear thermal expansion within a range between 30 and 380xc2x0 C. is xe2x88x9210xcx9c10xc3x9710xe2x88x927/xc2x0C., and xcex2-OH in an amount of 0.28/mm or more is contained.
Further, an Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallizable glass according to the present invention is characterized by having a property of being transformed into a crystallized glass in which xcex2-quartz solid solution is produced as main crystals, an average coefficient of linear thermal expansion within a range between 30 and 380xc2x0 C., is xe2x88x9210xcx9c10xc3x9710xe2x88x927/xc2x0C., and xcex2-OH in an amount of 0.28/mm or more is contained.
In the present invention, the amount of xcex2-OH is obtained by the following formula (1) by the use of an infrared absorption spectrum of the crystallized glass.
Amount of xcex2-OH={log(T3850/T3500)}/txe2x80x83xe2x80x83(1)
wherein
T3850: transmittance around 3850 cmxe2x88x921 
T3500: minimum transmittance in an absorption band around 3500 cmxe2x88x921 
t: thickness (mm) of crystallized glass at measurement of spectrum.